Intraluminal devices such as grafts and stents are known for treating stenosis, stricture, aneurysms and the like. These devices may be implanted either transluminally in a minimally invasive procedure or may be surgically implanted.
Such intraluminal devices provide a technique for expanding a constricted vessel or for maintaining an open passageway through a vessel. One common technique used to hold open a blocked or constricted vessel such as a blood vessel is to employ a vascular stent. Stents are implantable intraluminal devices typically formed of wire which may be radially expanded to hold open constricted vessels. Thus, wire stents are useful to prevent restenosis of a dilated vessel or to eliminate the danger of reocclusion of the vessel. In addition, wire stents can also be used to reinforce various lumen in danger of collapse. However, stents are not generally designed as conduits or bypass devices.
Intraluminal or endoprosthetic grafts, however, are designed as bypass devices which allow fluid flow therethrough. Often, these devices are percutaneously implanted within the vascular system to reinforce collapsing, partially occluded, weakened or abnormally dilated localized sections of, e.g., a blood vessel. Grafts may also be surgically implanted by anastomosis to replace a badly damaged portion of vessel.
Vascular grafts may be manufactured from a variety of bio-compatible materials. For example, it is well known to use extruded tubes of polytetrafluoroethylene (PTFE) as vascular grafts. PTFE is particularly suitable as it exhibits superior biocompatibility. PTFE tubes may be used as vascular grafts in the replacement or repair of blood vessels because PTFE exhibits low thrombogenicity. Further, expanded PTFE (ePTFE) tubes have a microporous structure which allows natural tissue ingrowth and cell endothelialization once implanted into the vascular system. This contributes to long term healing and graft patency.
Grafts formed of ePTFE have a fibrous state which is defined by interspaced nodes interconnected by elongated fibrils. The spaces between the node surfaces that are spanned by the fibrils are defined as the internodal distance (IND). The art is replete with examples of vascular grafts made of microporous ePTFE tubes useful as vascular grafts. The porosity of an ePTFE vascular graft is controlled by varying the IND of the microporous structure of the tube. An increase in the IND within a given structure results in enhanced tissue ingrowth, as well as, cell endothelialization along the inner surface thereof. Increasing the porosity of the tubular structure, however, reduces the ability of the graft to retain a suture placed therein during implantation and tends to exhibit low axial tear strength. In order to strike an effective balance between porosity and radial strength, multi-layer ePTFE tubes have been developed. The porosity of these multilayered tubes vary as between the outer and inner layers to achieve a composite structure having sufficient porosity for tissue ingrowth and cell endothelialization while still retaining sufficient radial strength.
It is known in the art to use stents in combination with vascular grafts and other endoprostheses. Stents may be positioned at one or both ends of a graft to support the graft within a portion of the vessel. Thus positioned, the stents help fix the graft to the vessel wall. In addition, stents serve to keep the lumen open and to anchor the graft in place. A single stent may also be employed in combination with a graft to allow the graft to "float" downstream toward the affected vessel. Once properly positioned, the single stent is expanded to anchor the graft in place.
Several techniques for securing one or more stents to a graft are known. For example, hooks or barbs extending from the stent have been used for securing stents to a graft. Alternatively, a stent may be sutured to a graft. Each of these techniques requires either specialized stent attachment means or secondary procedures to secure the stents to the graft.
Traditional stents have various shapes and sizes depending upon their intended function. For example, structures which have previously been used as stents include coiled stainless steel springs, helically wound coiled springs manufactured from an expandable heat-sensitive material, expanding stainless steel stents formed of stainless steel wire in a "zig-zag" pattern, cage-like devices made from malleable metal, and flexible tubes having a plurality of separate expandable ring-like scaffold members which permit radial expansion of a graft. Each of these devices is designed to be radially compressible and expandable so that it will easily pass through a blood vessel in a collapsed state and can be radially expanded to an implantable size after the target area of the vessel has been reached. Radial expansion and contraction of each of these causes associated longitudinal expansion and contraction of the stent.
Such expandable stents may be supported between the layers of a multi-layer tubular graft. The expandable stent would anchor and support the multi-layer tube within the lumen. Upon radial expansion, the stent would hold the graft outwardly against the inner wall of the lumen.
One example of such a graft-stent combination is shown in U.S. Pat. No. 5,123,917 issued to Lee et al. A stent-graft combination shown therein includes a plurality of separate scaffold members (stents) mounted between an inner tube and an outer tube forming the multi-layer graft. In one embodiment of this invention, the scaffold members are free floating within an intermediate pocket formed by the inner and outer tubes. In another embodiment, the scaffold members are adhesively affixed to the outer surface of the inner tube. In yet another embodiment of this invention, the inner and outer tubes are adhered to each other in such a manner that separate pockets are formed in which individual scaffold members are placed within each pocket.
In each of these different embodiments of the '917 patent, radial expansion of the scaffold member causes a change in the longitudinal expanse thereof. Thus, a drawback to the device shown in the '917 patent is that the net length of the scaffold member increases as the graft contracts. Accordingly, this increase in the net length of the scaffold member increases the stress forces on the graft as well as tends to delaminate the layers. Thus, these stress forces increase the likelihood that the inner tube will become separated from the outer tube and/or that the graft will tear upon expansion of the scaffold members.
Accordingly, it would be desirable to provide an improved intraluminal device, in particular, an ePTFE graft-stent composite device with improved radial strength that allows for the deployment of a stent and graft simultaneously with the stent already permanently positioned on the graft such that additional stress is not placed on the graft by the stent upon expansion.